spate-s3 0.1.0

Coordinated object-storage (S3) backfill source for the Spate pipeline framework: an elected leader plans the prefix into splits, workers lease them with fenced per-split progress in a coordination store, resume is drift-checked against ETag pins, and the job self-terminates when the plan completes. Applications should depend on the `spate` facade crate with the `s3` feature.